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engine testing
WHAT IS ELECTRIC PROPULSION?
Electric propulsion differs from
conventional chemical rockets in that
a strong electrical charge ionizes the
fuel. There is no combustion to create
the pressure inside a chamber for
generating thrust. Instead the magnetic
field accelerates the ions of atoms,
typically from an inert gas such as xenon
or argon.
Like chemical rockets there are a
variety of electric propulsion designs,
including Hall Effect thrusters such as
Hermes and Magneto Plasma Dynamic
AEROSPACETESTINGINTERNATIONAL.COM // JUNE
10,000 hours
of testing for NASA’s
Hermes rocket
18m
long vacuum chamber
for testing the
Hermes rocket
thrusters such as the VASIMIR. Other
types include the Gridded Ion Engine,
the High Efficiency Multistage Plasma
Thruster, the Pulsed Plasma Thruster,
the Quad Confinement thruster, the
Field Emission Electric Propulsion
(FEEP) thruster, Colloid and electrospray
thrusters and the Electrode-less
thruster. They all use a single propellant,
which can be a gas, a liquid metal or
a conventional propellant and use a
high-power electrical supply to heat and
accelerate the propellant.
how effectively the ions are being accelerated on the
central axis of the thruster. The Langmuir probe, which
can have up to four tips, detects the plume’s plasma
electron temperature, its electron density and the
electron potential of the plasma. The retarding potential
analyzer helps measure how well the ions are being
accelerated. “We also use a Wien filter spectrometer to
determine the composition of the plasma,” says Frieman.
For many years these probes were made without
standards as bespoke instrumentation that broadly met
the requirements for each customer. But eventually the
commercial use of such probes in the development of
satellites drove the need for agreed
standards. “Around 2016, the American
Institute of Aeronautics and Astronautics
put out a set of recommended standards
for all those probes,” says Walker. “Now
we can ensure that people around the
world are using the same probes or at
least processing the data in the same way.
It’s been a good move for the community.”
As well as inserting physical probes
into the path of the plume, optical
diagnostics that use lasers and infrared
(IR) high-speed cameras are used to
examine the plasma movements. “The
issue with the high-speed cameras is that
because the frame rate is so high, the
amount of light been detected between
one frame to the next is quite low,”
explains Walker. To overcome this, lasers
are used to excite the plume’s particles.
3 // Engineer Peter
Peterson prepares a high
power Hall thruster for
ground testing in a vacuum
chamber. (Photo: NASA
Glenn Research Center)
time is just not practical,” Frieman explains. “So, we have
computerized systems, checking telemetry, checking
against safety limits, making sure that everything is
okay.” This level of automation is a new aspect to the
laboratory and is being retrofitted to equipment.
DATA OVERLOAD
Running a rocket for so long generates a huge amount of
data. “You can easily generate a few gigabytes of data
during a test, so you can imagine if you were going to
keep an eye on it for a year, you can generate several
terabytes,” says Georgia Institute of Technology’s High-
Power Electric Propulsion Laboratory head, professor
Mitchell Walker.
At NASA’s Glenn Research Centre, engineers are
planning to increase the capacity of data storage systems
and added redundancy to the primary data archive in
preparation of Hermes testing. Glenn researchers are
deploying battery backup systems to make sure that we
have enough time to respond and ensure data is safely
preserved. Such measures will mitigate the risk of losing
data if a power loss occurs while the test is unattended,
believes Frieman.
PLUME POTENTIAL
The data comes from a suite of probes, lasers and highspeed
cameras. For the 10,000-hour test a number of
probes will be used by the NASA team. Those probes are
called Faraday, Langmuir and the retarding potential
analyzer. The Faraday probe allows researchers to
determine the exhaust plume’s shape, which will indicate
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